// Tamir Khason http://khason.net/
//
// Released under MS-PL : 6-Apr-09

using System;
using System.Collections;
using System.IO;
using System.Security.Cryptography;
using System.Text;
namespace AppEntity
{
    public partial class Common
    {
        public partial class Utility
        {
            /// <summary>Implements a 32-bits cyclic redundancy check (CRC) hash algorithm.</summary>
            /// <remarks>This class is not intended to be used for security purposes. For security applications use MD5, SHA1, SHA256, SHA384, 
            /// or SHA512 in the System.Security.Cryptography namespace.</remarks>
            public class CRC32 : HashAlgorithm
            {

                #region CONSTRUCTORS
                /// <summary>Creates a CRC32 object using the <see cref="DefaultPolynomial"/>.</summary>
                public CRC32()
                    : this(DefaultPolynomial)
                {
                }

                /// <summary>Creates a CRC32 object using the specified polynomial.</summary>
                /// <remarks>The polynomical should be supplied in its bit-reflected form. <see cref="DefaultPolynomial"/>.</remarks>
                [CLSCompliant(false)]
                public CRC32(uint polynomial)
                {
                    HashSizeValue = 32;
                    _crc32Table = (uint[])_crc32TablesCache[polynomial];
                    if (_crc32Table == null)
                    {
                        _crc32Table = CRC32._buildCRC32Table(polynomial);
                        _crc32TablesCache.Add(polynomial, _crc32Table);
                    }
                    Initialize();
                }

                // static constructor
                static CRC32()
                {
                    _crc32TablesCache = Hashtable.Synchronized(new Hashtable());
                    _defaultCRC = new CRC32();
                }
                #endregion

                #region PROPERTIES
                /// <summary>Gets the default polynomial (used in WinZip, Ethernet, etc.)</summary>
                /// <remarks>The default polynomial is a bit-reflected version of the standard polynomial 0x04C11DB7 used by WinZip, Ethernet, etc.</remarks>
                [CLSCompliant(false)]
                public static readonly uint DefaultPolynomial = 0xEDB88320; // Bitwise reflection of 0x04C11DB7;
                #endregion

                #region METHODS
                /// <summary>Initializes an implementation of HashAlgorithm.</summary>
                public override void Initialize()
                {
                    _crc = _allOnes;
                }

                /// <summary>Routes data written to the object into the hash algorithm for computing the hash.</summary>
                protected override void HashCore(byte[] buffer, int offset, int count)
                {
                    for (int i = offset; i < count; i++)
                    {
                        ulong ptr = (_crc & 0xFF) ^ buffer[i];
                        _crc >>= 8;
                        _crc ^= _crc32Table[ptr];
                    }
                }

                /// <summary>Finalizes the hash computation after the last data is processed by the cryptographic stream object.</summary>
                protected override byte[] HashFinal()
                {
                    byte[] finalHash = new byte[4];
                    ulong finalCRC = _crc ^ _allOnes;

                    finalHash[0] = (byte)((finalCRC >> 0) & 0xFF);
                    finalHash[1] = (byte)((finalCRC >> 8) & 0xFF);
                    finalHash[2] = (byte)((finalCRC >> 16) & 0xFF);
                    finalHash[3] = (byte)((finalCRC >> 24) & 0xFF);

                    return finalHash;
                }

                /// <summary>Computes the CRC32 value for the given ASCII string using the <see cref="DefaultPolynomial"/>.</summary>
                public static int Compute(string asciiString)
                {
                    _defaultCRC.Initialize();
                    return ToInt32(_defaultCRC.ComputeHash(asciiString));
                }

                /// <summary>Computes the CRC32 value for the given input stream using the <see cref="DefaultPolynomial"/>.</summary>
                public static int Compute(Stream inputStream)
                {
                    _defaultCRC.Initialize();
                    return ToInt32(_defaultCRC.ComputeHash(inputStream));
                }

                /// <summary>Computes the CRC32 value for the input data using the <see cref="DefaultPolynomial"/>.</summary>
                public static int Compute(byte[] buffer)
                {
                    _defaultCRC.Initialize();
                    return ToInt32(_defaultCRC.ComputeHash(buffer));
                }

                /// <summary>Computes the hash value for the input data using the <see cref="DefaultPolynomial"/>.</summary>
                public static int Compute(byte[] buffer, int offset, int count)
                {
                    _defaultCRC.Initialize();
                    return ToInt32(_defaultCRC.ComputeHash(buffer, offset, count));
                }

                /// <summary>Computes the hash value for the given ASCII string.</summary>
                /// <remarks>The computation preserves the internal state between the calls, so it can be used for computation of a stream data.</remarks>
                public byte[] ComputeHash(string asciiString)
                {
                    byte[] rawBytes = ASCIIEncoding.ASCII.GetBytes(asciiString);
                    return ComputeHash(rawBytes);
                }

                /// <summary>Computes the hash value for the given input stream.</summary>
                /// <remarks>The computation preserves the internal state between the calls, so it can be used for computation of a stream data.</remarks>
                new public byte[] ComputeHash(Stream inputStream)
                {
                    byte[] buffer = new byte[4096];
                    int bytesRead;
                    while ((bytesRead = inputStream.Read(buffer, 0, 4096)) > 0)
                    {
                        HashCore(buffer, 0, bytesRead);
                    }
                    return HashFinal();
                }

                /// <summary>Computes the hash value for the input data.</summary>
                /// <remarks>The computation preserves the internal state between the calls, so it can be used for computation of a stream data.</remarks>
                new public byte[] ComputeHash(byte[] buffer)
                {
                    return ComputeHash(buffer, 0, buffer.Length);
                }

                /// <summary>Computes the hash value for the input data.</summary>
                /// <remarks>The computation preserves the internal state between the calls, so it can be used for computation of a stream data.</remarks>
                new public byte[] ComputeHash(byte[] buffer, int offset, int count)
                {
                    HashCore(buffer, offset, count);
                    return HashFinal();
                }
                #endregion

                #region PRIVATE SECTION
                private static uint _allOnes = 0xffffffff;
                private static CRC32 _defaultCRC;
                private static Hashtable _crc32TablesCache;
                private uint[] _crc32Table;
                private uint _crc;

                // Builds a crc32 table given a polynomial
                private static uint[] _buildCRC32Table(uint polynomial)
                {
                    uint crc;
                    uint[] table = new uint[256];

                    // 256 values representing ASCII character codes. 
                    for (int i = 0; i < 256; i++)
                    {
                        crc = (uint)i;
                        for (int j = 8; j > 0; j--)
                        {
                            if ((crc & 1) == 1)
                                crc = (crc >> 1) ^ polynomial;
                            else
                                crc >>= 1;
                        }
                        table[i] = crc;
                    }

                    return table;
                }

                private static int ToInt32(byte[] buffer)
                {
                    return BitConverter.ToInt32(buffer, 0);
                }
                #endregion

            }
        }
    }
}
